A laser device consists usually of an active medium located within an optical resonator formed by two mirrors, resulting in a laser beam transmitted through one of these mirrors. One of the basic properties of laser beams is their small divergence angle which is particularly important in applications where such a beam is transmitted to long distances or focused to a small spot. Since the performance of the laser beam depends on its divergence angle such angle ought to be essentially constant under various operating conditions.
In many lasers, and particularly in solid-state lasers, the divergence angle is affected by the heating of the active medium and as a result depends on the operating conditions. In solid-state lasers the active medium is usually in the form of a cylindrical rod that is excited by absorbing the radiation emitted from a flashlamp. As a result of this excitation the rod is heated and needs cooling by a cooling liquid flowing over its surface. A combined effect of such heating and cooling is the formation of a radial temperature gradient across the rod, due to which the rod acts in the laser resonator as a lens with an optical power .phi. that is determined by the average heating power applied on the rod. In multimode lasers, this optical power determines the divergence angle .theta. of the output beam. In the case of a laser with a resonator formed by two plane-mirrors the divergence-angle may be calculated by the approximate formula: EQU .theta..perspectiveto.D.sub.o (.phi./L).sup.1/2 ( 1)
where D.sub.o is the diameter of the beam, L is the resonator length defined as the distance between the mirrors, and .phi. is the optical power of the heated active medium. As can be seen from Eq(1) a strong thermal lensing in the active medium tends to increase the divergence of the output beam. To overcome such increase in beam divergence, there is introduced into the resonator a negative (concave) lens which compensates the thermally-induced optical power of the active medium. The compensation is effective only for a specific value of .phi.. Any variation in the electrical voltage on the flashlamp or in the repetition rate of the flashes affects the thermal lensing of the active medium so that it is no more exactly compensated by the fixed concave lens.
As a result, the divergence of the output beam varies with variations of the operating conditions.
A possible solution to the problem of variation in rod thermal lensing is to introduce a dynamic compensator into the resonator. Such a compensator consists for example of a pair of lenses--one concave and one convex--with a variable spacing between them so that their common equivalent focal length is variable. The spacing between the lenses is varied according to the variation in operating conditions so that the rod thermal lensing is always compensated, thereby maintaining a fixed divergence of the generated beam. This solution however suffers from the inconvenience of using in the laser resonator moving optical elements which have to be adjusted according to operating conditions.
Another solution to the problem of variation in rod thermal lensing is based on the design of a resonator which is insensitive to variation in focal length of the pumped active medium. Several designs have been proposed for the realization of such resonators referred to as "dynamic stable resonators".
One of these designs proposed by Steffen et al in IEEE Journal of Quantum Electronics volume QE-8, page 239 (1982) uses a "semiconfocal resonator" in the form of two plane mirrors with the laser rod close to one mirror and the mirrors spaced by half the focal length of the rod's thermal lens. An obvious disadvantage of this design is the inconveniently long resonator.
Another design proposed by Chesler and Maydan in Journal of Applied Physics, Volume 43, page 2254 (1972) uses a "convex-concave resonator" in which there are one convex and one concave mirror. While operating successfully, the laser constructed according to this design is limited to low power operation because of the risk of damage to the convex mirror when the beam diameter is very small. This precludes the use of this design in high power Q-switched lasers.
Still another "dynamic stable resonator" design is the "telescopic resonator" used by Hanna et al and described in Optical and Quantum Electronics, 13, 493 (1981). In this design, a beam expander inserted into the laser resonator can be adjusted so as to make the resonator insensitive to variations in the rod's focal length. However, as in the "convex-concave resonator", in the "telescopic resonator" too, a small beam diameter is obtained on one of the mirrors so that the use in high power Q-switched lasers is precluded.
Another prior art design which is relevant to the present invention is the laser oscillator-amplifier configuration. In this configuration a low-power oscillator generates an oscillator beam which is amplified by the amplifier thus producing a high-power beam. The oscillator consists of an active medium located within the resonator defined by two mirrors. The amplifier consists of a second active medium external to the resonator. In such systems both active media are subject to thermal lensing which depends on the operating conditions, e.g. the pumping power or the pulse repetition rate.
The thermal lensing of each of the active media may be compensated separately by the use of optical means (lenses or mirrors). In the oscillator such a compensation is done for example by the insertion of a diverging (negative) lens somewhere between the mirrors or by making one of the mirrors with a convex reflecting surface. The thermally-induced optical power of the amplifier is similarly compensated by adding a diverging lens on either side of the active medium.
Usually, both active media are compensated for some specific operating conditions for example for specific pumping levels of the active media and for a specific pulse repetition rate. In such systems the active medium of the oscillator is usually compensated first so that the oscillator beam has an optimum beam divergence at some specific operating conditions for example at one particular repetition rate. Then, the amplifier is compensated by adding a lens designed for optimum beam divergence of the output beam at the same particular repetition rate. In this way, the laser system is optimized for one particular repetition rate. At higher and lower repetition rates the output beam becomes more divergent. Another drawback is the reduced energy and the poor stability of the laser at repetition rates lower than that for which it was optimized, resulting from the fact that the resonator becomes unstable at these repetition rates.